It has long been desirable to provide relatively inexpensive, high performance permanent magnets. The performance of such permanent magnets is determined by, for example, coercive force (H.sub.c) or coercivity, remanent magnetization (Br) or remanence and maximum magnetic energy product (BH).sub.max.
The search for satisfactory permanent magnets, has lead to permanent magnets composed of rare earth elements and the element cobalt, most significantly samarium-cobalt magnets. These have been prominent since the 1970's. Because of their unquestionable superiority in magnetic performance, the rare earth-cobalt magnets have governed over 20% of the present permanent magnet market. However, since rare earth powders are highly active in air and in some cases are highly ignitable, great care should be taken in preventing the oxidation thereof in their production. Many remedial means have been devised to overcome these problems. Some of them involve the use of anti-oxidants.
Motivated by the high cost and relative scarcity of samarium and cobalt, a new series of less expensive neodymium-iron-boron permanent magnets have been developed (neodymium is cheaper than samarium and iron is cheaper than cobalt). Among these the neodymium-iron-boron permanent magnets produced by the powder metallurgical method developed by Sagawa of Sumitomo Co., Japan in 1983 and those produced by a fast solidifying method developed by Croat of General Motors Co., U.S.A. are considered representative. However, the neodymium-iron-boron magnet powders are even more active in air than samarium-cobalt magnet powders and therefore the prevention of oxidation becomes much more critical. In the production of sintered magnets, the prevention of oxidation is even more difficult. One approach has been fabricating the magnets in a vacuum or an inert atmosphere. This method is effective in preventing some oxidation of magnet powders, however, complete prevention of oxidation in air is rather difficult and costly.
Phosphate or phosphoric acid in aqueous solution in combination with a minute amount of nylon and silicone oil have been used and taught as effective in preventing oxidation of magnet powders of average particle sizes from tens to hundreds of micrometers in the production of plastic magnets. The process is effective on plastic materials injection molded at about 200.degree.-230.degree. C. Typical references are U.S. Pat. No. 4,497,722 issued to Tsuchida et al. on Feb. 5, 1985 and Japan Laid-open Patent Application No. (Sho)60-188459 filed by Nakatsuka et al. and laid-open on Sept. 25, 1985. No reference to the prevention of the oxidation of powder metallurgical neodymium-iron-boron magnet powders has been disclosed.
Lubricants for the production of rare earth magnets such as Elvaoite, Microwax, Acrawax, Carbowax, stearic acid and stearate have been disclosed as being effective in preventing oxidation to a limited extent. Typical references are Japanese Laid-open Patent Application No. (Sho)61-90401 laid-open on Mar. 8, 1985 and U.S. Pat. No. 3,964,939 issued to Chandross et al. on Jun. 22, 1976. This approach is inapplicable to the prevention of the oxidation of powder metallurgical neodymium-iron-boron magnet powders due to its limited ability to prevent oxidation.
Some special dyes such as direct dyes, acidic dyes, alkaline dyes, hardening dyes, medium dyes, oily dyes and dispersing dyes which possess anti-oxidant characteristics are frequently used in the production of rare earth plastic magnets which are made or used at elevated temperatures. One typical reference is U.S. Pat. No. 3,892,600 issued to Smeggil et al. on Jul. 1, 1975. No reference to the prevention of the oxidation of powder metallurgical neodymium-iron-boron magnet powders has been disclosed.
Japanese Laid-open Patent Application No. (Sho) 60-244004 discloses the use of organopolysiloxane as the coupling agent in the production of plastic magnets to improve the mixing and bonding of the magnetic powder and the plastic components. Japanese Laid-open Patent Application No. (Sho)60-14406 discloses the use of titanate coupling agents in similar processes. No implication to prevention of oxidation in the production of sintered magnets has been disclosed.
Although there have been some instances in which the oxidation of magnetic powder in the production of plastic magnets has been successfully prevented by some anti-oxidants, there has not been a satisfactory method of preventing oxidation using anti-oxidants for the production of rare-earth magnets employing a powder metallurgical process.
It is therefore desirable to provide a method of effectively and economically preventing the oxidation of magnetic powders in the powder metallurgical production of rare-earth neodymium-iron-boron magnets, which method, or course, is also applicable to the production of rare earth-cobalt magnets.